The present invention relates to a multi-function sound system having an AM stereophonic receiving function. More particularly, the invention relates to a multi-function sound system which incorporates an AM-PM stereophonic type AM receiving function and other functions such as an FM receiving function and the like.
In an AM-PM type AM stereophonic broadcasting system, an AM stereophonic signal is transmitted in which a carrier signal is phase modulated by a sub signal and amplitude modulated by a main signal. There has been previously disclosed an AM-PM type AM stereophonic receiving system in which the envelope of a received signal of this type is amplitude detected to thereby obtain a main signal output, amplitude variation components of the received signal are removed by a limiter having an amplifying function, and then the resulting signal is phase detected with a phase-locked loop (PLL) circuit to thereby obtain a sub signal output. The main and sub signals are combined in a matrix circuit so as to obtain right and left channel signal outputs.
There has also been disclosed a so-called multi-function sound system which incorporates such an AM stereophonic receiving function and other operating functions such as, for example, an FM receiving function, a record disc reproducing function, and the like. When the multi-function sound system is, however, switched to a function other than the AM stereophonic receiving mode, problems occur. Specifically, since the limiter in the AM stereophonic receiver section has a large amplification factor and produces a substantially rectangular pulse signal, if a small level of noise is introduced into the input of the limiter, the limiter produces a high level pulse which adversely affects other circuits such as by induction or radiation. In addition, the free-running oscillator output from the voltage-controlled oscillator (VCO) in the PLL may also radiate to other circuits.